Modal interchange and advanced progressions

Beyond the seven diatonic chords

Diatonic harmony offers a key's seven chords. But almost every mature piece also reaches for chords 'from outside'. These are not mistakes - they are deliberate tools for colour and surprise. Here are the three most powerful mechanisms.

Modal interchange

Modal interchange means borrowing a chord from the parallel key of the opposite quality - most often from minor into major. In C major we can drop in F minor (from C minor) or ♭VII (B♭). The ♭VII chord and the minor iv ('the minor subdominant') are among the most common borrowed chords in pop and rock.

Secondary dominants

Any chord can briefly become a temporary tonic if we place its own dominant in front of it - a secondary dominant. The notation V/V ('the dominant of the dominant') in C major means D7, leading to G. This momentarily 'spotlights' the target and creates a stronger pull than a diatonic chord would.

Modulation

Modulation is a change to another key mid-piece. It works most smoothly through a pivot chord (a chord that exists in both keys) or through a secondary dominant that prepares the new tonic. Neighbouring keys on the circle of fifths modulate most naturally.

• Modal interchange: colour without changing key.
• Secondary dominant: a temporary aim at another chord.
• Modulation: a permanent move into a new key.

That closes the advanced tier. From the seven letters of the musical alphabet to modulation - you now have a complete map of harmony. The most beautiful part awaits: using all of this in real music.